It is known in the art of optical fibers to impress one or more Bragg gratings in the core of an optical fiber. A Bragg grating as is known reflects a predetermined wavelength-band of light incident thereon and passes the remaining wavelengths. As is also known, Bragg gratings have many uses such as for sensor devices and as components for fiber communications. They provide a wavelength-tunable reflective element which can be used as transducer elements in fiber sensors, as wavelength control devices for fiber, semiconductor, and solid state lasers, as wavelength division multiplexing (WDM) components in communication systems, as wavelength analyzers, as components in signal processing systems, and for other uses. Many of these devices would be greatly enhanced if the fiber grating element had a large wavelength tuning range.
A fiber Bragg grating is wavelength tuned (or changed) by stretching the fiber grating. One technique used is to attach the fiber grating to a piezoelectric stretcher (or tuner) which expands as a function of voltage applied to it or to wrap the grating around a cylindrical mandrel which expands when voltage is applied as described in U.S. Pat. No. 5,007,705 entitled "Variable Optical Fiber Bragg Filter Arrangement," to Morey et al. Numerous other stretching techniques have also been employed, as discussed in the aforementioned patent.
However, the amount that the fiber may be stretched (or tensile strained) and, thus, the maximum wavelength tuning range, is limited by the tensile strength of the fiber. In particular, when a Bragg grating is stretched the Bragg grating reflection wavelength change is about 1.2 nanometer(nm)/millistrain in the 1.55 micron wavelength reflection region. Typical communications-grade optical fibers and waveguides are made of Silica or Silicon Dioxide (SiO.sub.2) which has a Young's modulus of 1.02.times.10.sup.7 PSI. Therefore, for a typical optical fiber which is proof tested at 50 kpsi, a maximum safe long-term strain of approximately 1/2% ((.DELTA.L/L)*100; where L is the length of fiber stretched) can be applied without degrading the fiber strength which would eventually cause the fiber to break. This limits the maximum amount of tensile strain Bragg grating reflection wavelength tuning to about 5 nanometers.
Alternatively, fiber gratings have been tuned by thermal variation. In that case, the grating is heated which primarily causes the grating to expand and experience a change in refractive index. The change in Bragg reflection wavelength to temperature is approximately 0.011 nm/degree Celsius. The primary adverse effect of thermal tuning is degradation in the amount of reflectivity of the Bragg grating, which is caused by thermal annealing. Such degradation can greatly reduce the usefulness of the gratings. Depending on the particular fiber, fabrication techniques, fiber coating and grating requirements, significant grating degradation can occur at temperatures above about 200 degrees Celsius, thereby limiting practical tuning of the fiber grating to about 2 nanometers.
However, for many applications it is desirable to obtain a fiber Bragg grating which is tunable over as large a wavelength range as possible.